1. Technical Field
The present invention relates to a GaN-based semiconductor element, such as a GaN-based heterojunction field effect transistor or the like.
2. Related Arts
A development of a field effect transistor (FET) is progressed using materials for a gallium nitride (GaN) based semiconductor, especially using a GaN/AlGaN based semiconductor materials, because the GaN-based semiconductor has a wider band gap energy than that of a semiconductor using GaAs-based materials and has a high heat resistance as it shows a superior performance in a high temperature.
So far, there is disclosed a GaN-based high electron mobility transistor (HEMT) comprised of a GaN-based compound semiconductor for an FET using a GaN-based semiconductor in a published Japanese patent application No. 2006-173582 (hereinafter, it is described as a document 1). In such the GaN-based HEMT, a buffer layer is formed as required. Moreover, a carrier transport layer and a carrier supply layer are epitaxial grown in order thereunto. Furthermore, electrodes are formed thereunto.
Moreover, there is disclosed a structure in a published Japanese patent application No. 2003-059948 (hereinafter, it is described as a document 2) as another conventional technology, wherein a region of a GaN-based semiconductor for HEMT element is formed on a buffer layer comprising a multilayered structure including a first layer comprised of an AlN layer and a second layer comprised of a GaN layer are alternately layered on a substrate of silicon.
So far, the GaN-based HEMT structure is used for a horizontal element as disclosed in the above mentioned document 1 and 2. However, it is required to use a high resistance buffer layer, as it is necessary to reduce a leakage current flowing through the buffer layer for obtaining the buffer layer with a high withstand voltage at the time of forming an element for a high withstand voltage and a large current. Moreover, there are provided problems that an on-resistance is increased and consequently an element cannot help but be broken down due to occurring a current collapse (a current slump) by being fluctuated an electric potential of a semiconductor layer (the buffer layer) neighboring to a substrate at the time of performing a high voltage switching in the case of using a sapphire as an insulator for the substrate.